1. Field of the Invention
The present invention relates to archery and more particularly to bow sights. Still more particularly the invention relates to a dual beam laser targeting aid providing synchronized control of convergence and inclination of the beams for guiding bow elevation prior to shooting.
2. Description of the Problem
Arrow flight is an example of ballistic dynamics. Pure ballistic dynamics deals with the trajectory of projectiles which generate no lift and have no continuing source of thrust. Ballistics considers the effects of gravitational fields and allows for aerodynamic drag. Under these conditions projectiles follow a decaying parabolic trajectory with the rate of velocity decay being highest where the velocity is greatest. Accordingly, bow archery has never been a matter of simply taking careful aim directly at a target and loosing the bowstring to send an arrow toward its mark. As long as the archer is not shooting vertically, the archer must estimate the range to the mark in order to give the arrow an initial upward velocity relative to his or her line of sight to the mark. The archer does this by elevating the bow to aim the arrow above the mark.
The degree to which the archer elevates the bow is complicated by a number of factors. An archer is trained to shoot from a full draw. For a given bow, a fill draw transfers a fixed amount of energy to the bow which is transferred to the arrow upon loosing the bowstring. Thus, a heavier (more massive) arrow will accelerate less than a lighter (less massive) arrow. An arrow with a larger arrowhead or fetching another arrow will generate correspndingly more drag at a given velocity, contributing to a greater decay in velocity. An archer compensates for all of these factors when elevating a bow for a shot.
Archers have long used sights to help aim arrows. Given the differing flight characteristics of arrows and the different energy capacity of different bows, a bowsight will typically be matched to a bow and arrow combination. Lasers have been extensively applied to improve the effectiveness of sights, particularly with respect to estimating the proper elevation of the bow. U.S. Pat. No. 5,782,002 to Reed and U.S. Pat. No. 5,495,675 to Huang are good examples. Both patents teach bowsights incorporating a laser. The laser is oriented to emit its light beam in the direction of arrow flight from the bow. In both patents the laser is positioned above the arrow rest and the inclination of the laser can be adjusted so that the beam illuminates the desired mark when the archer has elevated the bow to compensate for the archer""s estimated range to target. Reed provides a cam based adjustment mechanism for setting laser inclination. However, the effectiveness of the Reed and Huang devices depends upon the archer""s correct in situ estimation of the range to the mark and a correct estimation of the arrow""s trajectory.
Laser sights exist for actually measuring range, such as Hines et al., U.S. Pat. No. 4,753,528. The Hines"" device is an electronic range finder incorporating a laser and a photosensitive detector for sampling reflected light. Such devices are illegal for hunting in several States and are not permitted at contests under applicable archery competition rules. It remains a problem to supply a bow sight which will guide an archer in correctly elevating his or her bow for hitting a mark without actively measuring the range to the target.
It is an object of the present invention to provide a laser bowsight aid which guides an archer in correctly elevating the bow to hit a mark with an arrow shot from the bow.
It is another object of the present invention to provide an adjustable bowsight aid which may be calibrated for arrows of differing flight characteristics and bows of differing energy capacities.
It is still another object of the present invention to provide bowsight aid which is light, robust and compact.
The present invention provides a sighting aid for use primarily with a short range, low projectile velocity weapon, preferably an archery bow. The sight comprises projectors for light beams, preferably provided by two lasers. In the preferred embodiment, both lasers are positioned, vertically aligned, above the arrow rest of the bow. The lasers are oriented to direct their beams in the general direction of flight of an arrow to be shot from the bow. The lasers are mounted on pivoting arms, diverging slightly outwardly from the vertical plane of the bow. The arms pivot on axes perpendicular to the plane of the bow, allowing the inclination of the lasers relative to the direction of arrow flight, and relative to one another, to be adjusted. The levers ride on the perimeter cam surfaces of a rotatable cam wheel. Convergence of the lasers is controlled by giving one of the perimeter cam surfaces a variable radius, while the second lever rides against a constant radius cam surface. Thus the angle of convergence of the lasers changes as the cam wheel rotates. The range to convergence of the beams is a fixed function of cam wheel position. The inclination of the two lasers is synchronously controlled by raising and lowering the cam wheel. This is done by providing a second, variable radius cam mounted on the cam wheel which rides against a brace fixed to a frame supporting the wheel. As the cam wheel rotates, the height of the cam wheel varies as a function of its radial position as different sections of the variable radius cam come into position on the fixed brace. The second variable radius cam, or declination control cam, is user adjustable, so that the degree of declination of the lasers for a given cam wheel position may be changed and may be calibrated with the range to convergence of the beams for the cam wheel position.
In use, the laser sighting aid produces two horizontally aligned illuminated dots on the target when the bow is correctly elevated. To effect this result, the convergence of the laser beams and the declination of the laser beams is calibrated. As described above, the trajectory of an arrow shot from a bow is a decaying parabola. Every point in the trajectory of the arrow will lie below a straight line projection of the arrow""s initial direction of flight. Lasers positioned with one above the other, and with both lasers above, but vertically aligned with, an arrow resting in the bow, can be oriented (with a declination) to intercept one another (converge) to intercept the arrow""s trajectory at a single point. With the ranges to convergence of the laser beams fixed as a function of cam wheel position, the declination of the lasers for each cam wheel position is adjusted to intercept the expected trajectory for an arrow by adjusting the declination control cam.
Typically, declination of the beams is calibrated for the ranges to convergence of the beams by trial and error. The declination control cam is step wise adjustable, based on a series of radially spaced, independently adjustable allen screws. An archer calibrates the system for a particular bow and arrow combination by taking aim at a series of targets, spaced to correspond to the changes in range to convergence for each available step in the declination control cam. Declination is adjusted at each range until the archer achieves consistent hits at the proper height for the range to target.
Additional effects, features and advantages will be apparent in the written description that follows.